Non-volatile memory devices are well known in the art. For example, a split-gate memory cell is disclosed in U.S. Pat. No. 5,029,130 (which is incorporated herein by reference for all purposes). This memory cell has a floating gate and a control gate disposed over and controlling the conductivity of a channel region of the substrate extending between source and drain regions. Various combinations of voltages are applied to the control gate, source and drain to program the memory cell (by injecting electrons onto the floating gate), to erase the memory cell (by removing electrons from the floating gate), and to read the memory cell (by measuring or detecting the conductivity of the channel region under the floating gate to determine the programming state of the floating gate).
The configuration and number of gates in non-volatile memory cells can vary. For example, U.S. Pat. No. 7,315,056 (which is incorporated herein by reference for all purposes) discloses a memory cell that additionally includes a program/erase gate over the source region. U.S. Pat. No. 7,868,375 (which is incorporated herein by reference for all purposes) discloses a memory cell that additionally includes an erase gate over the source region and a coupling gate over the floating gate. See also U.S. Pat. Nos. 6,747,310, 7,868,375, 9,276,005 and 9,276,006 (which are also incorporated herein by reference for all purposes).
Because the problem of shrinking the lithography size thereby reducing the channel width affects all semiconductor devices, a Fin-FET type of structure has been proposed. In a Fin-FET type of structure, a fin shaped member of the semiconductor substrate material connects the source region to the drain region. The fin shaped member has a top surface and two opposing side surfaces. Current from the source to the drain regions can then flow along the top surface as well as the two side surfaces. Thus, the surface width of the channel region is increased, thereby increasing the current flow, without sacrificing more semiconductor real estate, by “folding” the channel region into two side surfaces, thereby reducing the “footprint” of the channel region. Non-volatile memory cells using such Fin-FETs have been disclosed. Some examples of prior art Fin-FET non-volatile memory structures include U.S. Pat. Nos. 7,423,310, 7,410,913, 8,461,640, and 9,634,018. However, these prior art Fin-FET structures have disclosed using the floating gate as a stack gate device, or using trapping material, or using SRO (silicon rich oxide) or using nanocrystal silicon to store charges, or other memory cell configurations that are either too simplistic for memory cells having more than 2 gates or too complex for the number of gates at issue.
A number of problems have been discovered by the inventors when scaling memory cells down in size. Ultra-thin polysilicon or amorphous silicon film deposition and doping techniques are complex and often suffer from insufficient and non-uniform doping combined with structural non-uniformities. Ballistic electron transport in ultra-thin polysilicon floating gates leads to programming issues (difficulties to capture the hot electrons in ultra-thin floating gates). Integration of the control gate on top of the floating gate results in thick poly stack posing serious process integration challenges for advanced CMOS technologies (CMP planarization steps and following advanced lithography steps used in high K metal gate process flows). Capacitive coupling between neighboring floating gates is drastically increased with horizontal scaling. This results in strong cross-talk effects and requires complex management by design (read current of the cell becomes dependent on the charge state of the neighbor cells). The scaling of the planar floating gate memory cells is limited by reduction of the read currents related to transistors width scaling. Lower read currents penalize the access times and require complex design techniques to meet high speed access time specifications. Planar floating gate architecture doesn't allow for efficient control of the sub-threshold leakage of the floating gate and select transistors at advanced technology nodes, resulting in high background leakage from the unselected cells sharing the same bit line with selected cell.